Archive for January, 2012

January 29, 2012

How large were the first stars in the universe?

Since it is currently, and for the foreseeable future, not possible to actually observe what the first stars in the universe were like when they formed, the only way to answer this question is by detailed calculations from first principles. In other words, by computer simulations. Until very recently, such simulations couldn’t be very conclusive, since they simply couldn’t handle the amount of detail required. But they indicated that the first stars may typically have been very massive – perhaps often 100 solar masses each.

However, the very latest simulations, which can take advantage of the most powerful supercomputers now available in order to do more detailed calculations, indicate that a typical first generation star may have been less than half as massive as previously indicated.

It’s much harder than one might suppose to simulate, in detail, the formation of stars in the very early universe. Ten years ago the best that could be done was to simulate the process just up to the point where a single clump of dense, hot gas only about 1% as massive as the Sun has formed. That’s really only the first part of the process, and what happens after that is crucial in order to figure out what the typical size of one of the first stars in the universe was.

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January 15, 2012

Primordial galaxy cluster is earliest ever seen

At what point did stars, galaxies, and galaxy clusters make their first appearance in the universe? There’s now good evidence that galaxy clusters were starting to form about 650 million years after the big bang. So galaxies must have begun forming earlier than that, and the first stars even earlier.

The evidence consists of the discovery that a known bright galaxy at z≈8 has 4 dimmer companions within a radius of about 10 million light-years. The known galaxy was originally found using the Wide Field Camera 3 (WFC3) of the Hubble Space Telescope, by means of the Lyman-break technique. The was done as part of a search for z≈8 galaxies, known as the Brightest of Reionizing Galaxies (BoRG) survey. The region in which the bright galaxy was found was designated BoRG58.

The Lyman-break technique is based on the fact that photons having wavelengths less than the Lyman-α length of 121.6 nm (medium ultraviolet) are easily absorbed by neutral hydrogen gas. Consequently, most of the light from hot young stars is extinguished by neutral hydrogen, since such stars are brightest at wavelengths shorter than Lyman-α. The wavelength of Lyman-α photons emitted at z≈8 is stretched by a factor of z+1 to 1094 nm, in the infrared range. So in order to identify bright galaxies at z≈8, images are made through a series of filters that pass photons on either side of 1094 nm. Objects that seem to disappear in images made through filters that pass only light with wavelengths below 1094 nm are probably at the desired redshift.

Four bright objects presumed to be at z≈8 were identified early in the survey process. Computer simulations have indicated that at that redshift the most massive galaxies (which are presumably the brightest) should have a number of less massive, less luminous companions. So the search was extended around each of the four candidates, using longer exposure times to detect fainter objects. 17 less luminous objects were detected with substantial confidence. Field BoRG58, which had the best original candidate, turned out to have 4 less luminous companions of the candidate at the best level of confidence.

It hasn’t yet been possible to obtain direct spectroscopic evidence for the actual redshift of the detected objects. This is probably because so much of the available light from those objects has been extinguished by neutral hydrogen. However, the detection of fainter z≈8 dropout objects near the original candidate provides additional evidence that the objects are not spurious dropouts (which can occur with low probability).

The computer simulations suggest that the halo mass (which includes dark matter) around the brightest object should be in the range 400 to 700 billion M. (The Milky Way’s halo is estimated to be more than 1 trillion M.) There should also be additional smaller halos nearby about 100 billion M, hosting the fainter dropouts. By the present time the system should have grown into a cluster with halo mass 200 trillion M, a fairly typical cluster size.

Further reading:

Hubble Pinpoints Furthest Protocluster of Galaxies Ever Seen

CU-led study pinpoints farthest developing galaxy cluster ever found

CU-Boulder finds farthest galaxy cluster ever seen

Hubble spies earliest galaxy cluster ever seen

Hubble shows images from record-breaking 13.1 billion light-years

Overdensities of Y-dropout Galaxies from the Brightest-of-Reionizing Galaxies Survey: A Candidate Protocluster at Redshift z≈8

January 15, 2012

Two galaxy clusters in an advanced stage of collision

DLSCL J0916.2+2951 is a recently discovered pair of galaxy clusters in an advanced stage of collision. Its redshift is z=0.53, corresponding to a distance of 5.2 billion light-years. The two clusters began to collide about 700 million years previously, making this the most advanced cluster collision discovered – from 2 to 5 times farther along than previous discoveries (of which there have been only 4 that are comparable). The 2 clusters together span a distance of about 6 million light-years.

The clusters have already passed through each other, though they will eventually merge completely. The galaxies and dark matter within each cluster were relatively unaffected by the encounter. However, the intergalactic gas contained in one cluster has interacted strongly with that of the other. Consequently, much of the gas has been separated from its original cluster. It now lies between the two original clusters and has a temperature around 6 million K – 1000 times hotter than the surface of the Sun. As a result, the gas is a strong emitter of X-rays whose photons have energies in the 0.5 to 2.0 keV range.

The amount of dark matter within each of the colliding clusters can be estimated by the technique of weak gravitational lensing. This is based on the fact that a very massive object, such as a galaxy or cluster of galaxies, bends light from objects on the far side of the massive object along the line of sight. The concentration of mass distorts the shape of the more distant objects – turning a circle into an ellipse, for example. A statistical analysis of the observed shapes, compared to what would be seen in the absence of the gravitational lens, yields an estimate of the amount of mass in the lens.

The total mass of one cluster is estimated to be about 2×1014 M and the other is 3×1014 M – a total of about 500 trillion solar masses for the two clusters together. Of that, 86% is dark matter, 12% very hot gas, and 2% visible stars. This ratio of dark matter to ordinary matter is close to the average for the universe as a whole – so this is a further confirmation of the abundance of dark matter.

Since the dark matter halos of the two original clusters have mostly passed through each other without combining the way that the intergalactic gas did, it has been possible to estimate that the probability of self-interaction between dark matter particles is relatively small. This “cross section” value can be used in simulations of how the universe has evolved on a large scale.

Further reading:

When galaxy clusters collide

Discovery of a Dissociative Galaxy Cluster Merger with Large Physical Separation

January 7, 2012

A hyperactive young galaxy

Active galaxies contain a supermassive black hole (SMBH) that causes vigorous radiation of electromagnetic energy as a result of rapid accretion of gas and dust. While almost all galaxies except dwarfs contain an SMBH in the center, active galaxies are rare – fewer that 1% of galaxies in the present universe. A very few active galaxies contain two active SMBHs. Even fewer have three. Before the latest discovery, only two examples had ever been documented in the literature.

The most recent example was so obscure it doesn’t even have a name. This example is unusual in other respects as well. It is quite distant, having a redshift of z=1.35. That means its light has taken 8.92 billion years to reach us. We see it as it looked about 4.75 billion years after the big bang. Most galaxies at that time were fairly mature. Not this one. It seems to be quite young and irregular in form, with 4 separate components. Three of those appear to contain active SMBHs.

Some of the details about this galaxy are not known very precisely. It’s (barely) possible that one or more of the apparent active SMBHs are actually bulges hosting very active star formation. But the spectroscopic evidence is heavily against that.

The three SMBHs are not especially large as such things go. The masses could be as much as 3.1×106 M, 1.0×107 M, and 1.2×107 M. However, these are upper bounds, and the actual masses could be only 20% as large. For comparison, the Milky Way’s SMBH is 4.2×106 M, but many SMBHs are in the 109 to 1010 M range.

There are several intriguing questions about this object. The first: how did it form? Is it a 3-way merger of smaller galaxies of similar size, each of which had its own SMBH already? In the modern universe, such mergers are very rare. The evidence is that they were also rare in the era of 5 billion years after the big bang. It’s a lot more likely that this galaxy is still rather young and in an early stage of formation.

If that is the case, then there are very interesting questions about how the object can have three active nuclei.

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January 4, 2012

Some supermassive black holes are much more super than others

Supermassive black holes (SMBHs) can get to be pretty large. Astrophysicists don’t really know what the upper limit is, if any. But before some recent research, the mass of the largest SMBH yet determined was 6.3×109 M (solar masses). That value is known fairly precisely, since the SMBH is in the nearby giant elliptical galaxy M87, which is a mere 53 million light-years away.

The latest research has identified two substantially larger SMBHs, but the masses are known less precisely, since the objects are a lot farther away. One SMBH is in NGC 3842 and has estimated mass of 9.7×109 M, at a distance of 320 million light-years. The other is in NGC 4889. Its mass is known considerably less precisely but may be more than twice that of the SMBH in NGC 3842. (2.1×1010 M is the midpoint of the possible range.) It’s 336 million light years away. Both of these galaxies are also giant ellipticals. The uncertainty in the SMBH mass is much larger for NGC 4889 than for NGC 3842 because the mass estimates are based on the velocities of stars very close to the SMBH, and the uncertainties of velocity measurements in the former case were more than in the latter.

The establishment of new records for directly measured SMBH masses is actually not the most interesting aspect of the new research. (Although the amount of media attention to the results might lead one to think it was.) One thing that’s more interesting is that a fairly straightforward method of estimating SMBH mass can be used out to a distance of several hundred million light-years with present technology.

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